Resistance welding utilizes the flow of electrical current to permanently join two or more overlapping pieces of metal together. The metallic workpieces are placed in an overlapping position between two electrodes. The electrodes are then forced together until they contact the workpieces therebetween at a pressure sufficient to provide a good electrical contact. An electrical current is induced to flow from one electrode, through the metal to be joined, and into the other electrode. The workpieces act as a conductor in the electrical circuit and resistance to the flow of current at the interface between the metals generates heat. Based on the amount of current which flows and the length of time it is allowed to flow, the metal positioned between the electrodes is transformed into a molten state which flows together and, when cooled, forms a weld "nugget" permanently bonding the pieces together.
During the weld process, however, the electrode tips or caps, usually formed of a highly conductive metal or alloy, also increase in temperature due to the passage of current therethrough as well as heat conduction from the workpieces. Without proper cooling of the tips, this can result in excessive tip wear, deformation of the tips, sticking of the tips to the workpiece or even tip melting, all of which contribute to increased maintenance and poor weld quality. To cool the electrode tips, a system which creates a flow of cooling fluid, usually water, to an interior cavity formed within the weld tip is typically employed.
A resistance welding apparatus, having a conventional weld tip cooling system is shown generally in FIG. 1. The apparatus consists of a weld gun 10 which includes a movable arm 12 and a fixed arm 14, each having an electrode holder 16 to hold an electrode 18 having an electrode tip 20. Typically, pneumatic pressure is applied to movable arm 12 to force that arm toward fixed or floating arm 14 and, hence, to force the electrodes 18 together to contact the workpiece 22 positioned therebetween at a predetermined pressure and for a predetermined time period. During the time in which the electrodes 18 contact workpiece 22, the flow of both electrical current and cooling water to the weld tip 20 is induced. Upon retraction of the movable arm 12 and concurrent removal of the electrode tips 20 from the workpiece 22 (or sometime therebefore) the flow of current and/or water is disrupted.
A water deflector tube 28 is employed within the electrode 18 to direct the flow of water to and from the weld tip area. The cooling water passes from the water inlet port 24 through an interior channel 30 in the water deflector tube 28 to a small hollow cavity 32 formed in the interior of the weld tip 20. The water circulates within the cavity 32 and then is forced out through an outlet channel 34 to the water outlet port 26.
A means for inducing and stopping the flow of water to the electrode tip, conventionally in the form of a system of water supply/return valves and manifolds (not shown), is housed external the gun 10. This water supply/return system is usually connected to inlet and outlet ports 24 and 26 on the gun 10 by hoses 36 and 38. Functions such as shutting off the water flow to the tip if the tip should break off can also be performed with the conventional system, commonly accomplished by sensing a pressure differential between water in the supply and return hoses. This is useful as the tip may be manually removed from the electrode for replacement or repair or may develop a hole or break off due to high heat or other problems.
However, there exist numerous disadvantages to this type of configuration wherein the means for controlling water flow is housed external to the gun. These disadvantages include a high degree of complexity, a large number of parts and relatively high cost.